During the production of phenol, not only is phenol produced but a co-product is alpha-methylstyrene which must be separated from phenol and recovered. In general, phenols are prepared by the oxidation of an alkyl-substituted aromatic hydrocarbon and the subsequent acid cleavage of the resulting alpha hydroperoxy derivative thereof to form a reaction mixture comprising a phenol, a ketone and an unreacted alkyl-substituted aromatic hydrocarbon. The acid cleavage is generally effected in the presence of an aqueous acid catalyst, usually 50-98% sulfuric acid in aqueous solution, and preferably at least 70%, or in the presence of an aqueous hydrochloric or perchloric acid solution. The recovery process of the present invention is particularly suited for use in conjunction with a process wherein phenol is prepared by the air oxidation of cumene and the subsequent sulfuric acid cleavage of the resulting cumene hydroperoxide to form a reaction mixture comprising phenol, acetone and unreacted cumene. In addition to the principal products, varying amounts of by-products are formed such as mesityl oxide, alpha-methylstyrene, methyl-benzofuran, p-cumylphenol, phenyldimethylcarbinol, acetophenone and higher molecular weight phenolics.
In the initial stage of recovering phenol from the acid cleavage reaction mixture, the acidic reaction mixture is initially neutralized, either directly by the addition of caustic followed by the separation of the organic phase from the aqueous phase which contains the sodium salt of the acid catalyst, or indirectly by contact with an ion exchange resin. In any case, the resulting neutralized reaction mixture is fed to a distillation column, commonly referred to as a crude acetone column, at conditions to effect a crude separation of those materials boiling below phenol whereby an overhead fraction is recovered comprising substantially all of the acetone and lower boiling by-products, as well as a substantial portion of the water and unreacted cumene. Acetone is subsequently recovered, as is cumene, by the further distillation of the crude acetone column overhead, the cumene being recycled to the oxidation step.
The bottoms fraction recovered from the crude acetone column, containing phenol, alpha-methylstyrene and heavy by-products as well as the balance of the water and unreacted cumene, is typically treated for the separation of heavy by-products and thereafter fed to a distillation column, commonly referred to as a cumene or alpha-methylstyrene column. The last-mentioned column is operated at conditions to separate an overhead fraction comprising water and cumene from the higher boiling phenol product. The phenol, recovered as the bottoms fraction, further contains impurities such as acetophenone, mesityl oxide, methyl benzofuran, hydroxy acetone, alpha-methylstyrene and a small amount of residual cumene for example. The phenol is introduced into a phenol stripper column with water to fractionate the lower boiling components consisting mainly of alpha-methylstyrene and to strip out the hard-to-fractionate trace amounts of contaminates such as methyl benzofuran and mesityl oxide; and to produce a bottoms stream containing phenol and heavier boiling components which may be further processed as required. The overhead stream from the phenol stripper column contains water, phenol, methyl benzofuran, alpha-methylstyrene and lower boiling organic components. A condensed aqueous stream containing phenol is refluxed to the phenol stripper column and a condensed net organic phase containing alpha-methylstyrene is recovered. Because of the formation of a phenol-water azeotrope, a significant amount of phenol leaves with the stripper overhead vapor which is largely comprised of water vapor and the alpha-methylstyrene and the lower boiling hydrocarbons. The resulting condensed organic phase containing mainly alpha-methylstyrene is scrubbed with caustic to remove phenol. The spent caustic from the scrubbing operation is subsequently treated with sulfuric acid in order to recover the phenol for recycle to the process. High phenol content in the mainly alpha-methylstyrene phase translates to increased consumption of caustic and sulfuric add, and burdens the associated equipment.